This invention relates generally to measurement instruments and more specifically to a device for use to measure warpage of a surface.
Various devices and methods have been developed for measuring warpage, or conversely flatness, of a surface. Typically, these measurement devices are used in a controlled environment. As a result, items requiring measurement must be transported to the measuring device. In many instances, however, it is desirable to have a portable measurement device which is easily transportable to the item, either because the item is not easily transported or unavailable for transport; i.e., an in-process item. For example, in the manufacture of corrugated webs or sheets, a transportable measurement device is desired which can be easily and quickly used for measuring in-process warpage.
In one specific example, in-process warp measurements are desired in the manufacture of corrugated cardboard to optimally adjust the amount of moisture content in the cardboard so as to minimize warpage resulting during its manufacture. One prior art warp measurement device employs a rule having a predetermined length which is placed across a portion of the cardboard. If the cardboard is warped, only the ends of the rule contact the cardboard thereby leaving a gap between the central portion of the cardboard and rule. The measured gap provides an indication of the warpage as a function of the predetermined length of the rule. Typically, a predetermined threshold level of warpage is acceptable in the manufacturing process. This predetermined level of warpage is expressed as a quality index; i.e., a dimensionless number such as "warpage per 12 inches of sheet." As such, it is desirable to have a portable warp measurement device which accurately measures warpage for a given length or width of sheet rather than as a function of the length of the rule, thereby facilitating immediate comparison with the quality index.
Current warp measurement devices are deficient in that they do not accurately measure warpage as a function of the length or width of sheet. For example, if warpage is measured using the above-described rule having a predetermined length of 12 inches, the indicated warpage will be in error since the actual length of the warped cardboard corresponding to the 12 inch rule will be slightly longer than 12 inches. Further, this error creates a false high reading of warpage. As a result, when comparing the measured warpage using prior art devices to a desired quality index, further correction factors must be applied to the measured warpage to provide an accurate indication of warpage corresponding to the quality index.
Therefore, an improved warp indicating device is desired. Such a device should provide an accurate indication of warpage for a given length or width of sheet. Such a device should be rugged so that it can be reliably used to measure warpage of a surface in a manufacturing environment. Such a device should also be inexpensive to manufacture to reduce cost and, thereby, increase its availability for use.